A fine pattern typically is formed on a generally very thin film on silicon, e.g., a silicon oxide or photo resist film over silicon, and the quality of the integrated circuit (IC, hereinafter) depends on the precision of the pattern's film thickness and the pattern line widths.
Hence for satisfactory production control of ICs a device for line width measurement is required. Currently available devices for this purpose typically utilize a photoelectrically produced photo image of the pattern of interest. The pattern is formed on a thin film, hence for accurate detection of a pattern edge for determining the line width it is necessary to accurately discriminate among minute differences in photo intensity. With such a line width measuring device therefore, its performance is determined by how accurately it is possible to detect a pattern edge from minute photo-intensity differences between photo images obtained from the pattern.
A known line width measuring device of this type uses a photoelectric microscope, as shown in FIG. 1, wherein 1 designates a microscope, 2 a specimen of the mask or wafer set on the microscope, 3 a photo-electric conversion element and 4 a slit which is driven synchronously with the photo-electric conversion element 3. Element 6 is a current-voltage converter comprising an amplifier, and 7 is digital counter. Signal processing means 8, e.g., a microcomputer which also controls a stepping motor 5, computes a photo-intensity signal received from the current-voltage converter 6, computes the line width, and outputs the result either to a display 9a or to a printer 9b. 8a is a keyboard to control the various operations.
A line width measuring device so using a photo-electric microscope measures the pattern's line width as follows:
The specimen 2 that is to be measured is first put on the specimen mount of the microscope 1. Watching through the binocular eyepiece 10, the user then moves pattern 12 in the direction indicated by the arrow to a position, for example, on the righthand side of the vertical line of the microscope's cross-hairs 11. See FIG. 2. Signal processing means 8 is then started. Slit 4 and photo-electric conversion element 3 are driven intermittently in one direction by stepping motor 5 controlled by the signal processing means 8, the image signal I of the pattern 12 is outputted stepwise, and the photo-intensity signal K so obtained is amplified by current-voltage converter 6. The photo-intensity signal, K, is compared with a predetermined reference level signal in the signal processing means 8 for the line width of the pattern 12 to be computed thereby, and the result is displayed or printed in units of 0.05 .mu.m.
It is also possible to use an image pickup tube instead of the combination of the slit and the photo-electric conversion element. In this case, however, the distortion of the image displayed on the tube must be less than 0.2%, hence compensation of the distortion, improvement of stability, etc., will be necessary in practical use. In yet another known method the scanning of the specimen's surface is done with a fine laser beam for detecting the scattering of rays due to pattern edges to thereby measure the line width.
In the above-mentioned prior art, involving scanning of the specimen's surface by a combination of a slit and a photo-electric conversion element, the image signal is outputted stepwise during scanning. Hence shortening of the measuring time is not feasible, particularly when a plurality of scans must be made for improved precision. As to the prior art in which the specimen's surface is scanned by the laser beam, the laser beam scanning optical system becomes complicated and limits the productivity of such device while resulting in increased production costs.
A need, therefore, exists for a simple, inexpensive and precise apparatus and method for measuring line width in very fine patterns.